Horseradish Mashed Potatoes with Caramelized Onions

¼ cup plus 6 tblsp butter
3 ½ pounds potatoes, but in to1 inch pieces
2 ¼ lbs white onions, thinly sliced
6-8 tbsp milk
3 tbsp balsamic vinegar
¼ cup horseradish
2 tsp chopped fresh thyme

Melt ¼ cup butter in large skillet over medium heat. Add onions, sauté until deep golden, about 30 minutes. Add vinegar and thyme. Reduce heat to low, sature 4 minutes. Season with salt and pepper.

Meanwhile cook potatoes in large pot of salted water, til tender, about 15-20 minutes.

Drain, return to pot and mash til smooth. Add 6 tbsp milk, horseradish and 6 tbsp butter, stir to blend. Thin with milk if necessary. Season with salt and pepper to taste.

Transfer potatoes to serving dish, spoon onion mixture over potatoes. Serve hot.

Serves 6